The Sun in a Bottle - Part II: Keeping the Sun at Bay

 

Welcome back everyone! In this second part of my article about fusion reactors, I am going to talk about the juicy stuff: the reactors itself! Let’s get right into it!

For an efficient fusion reaction, the energy output needs to be greater than the energy input.

As I said in the first part of the article, nuclei need to be close enough to overcome the electrostatic repulsion between them.

Less energy is required for lighter nuclei to fuse because the fewer the nucleons (protons), the lower the repulsive force.

“Binding energy per nucleon”

So, what’s the element with lowest charge, and thus, with the fewest number of protons?

It’s hydrogen, which is the same element that ignites fusion reactions in the core of a star!

In a fusion reactor, two isotopes of hydrogen, deuterium, and tritium, are used as fuel, because they reach fusion conditions at lower temperatures compared to other elements, and release way more energy than other fusion reactions.

The plasma where the fusion reaction happens needs to be confined in a discrete volume to achieve fusion power.

So, how is plasma confined?

There are two fundamental methods: magnetic confinement and inertial confinement.

Magnetic confinement: the Tokamak

The first fundamental type of fusion reactor is the Tokamak, a device that uses a powerful magnetic field to contain plasma in the shape of a torus.

“Cross-section of the ITER magnetic fusion reactor”

The toroidal configuration was conceptualized in the 1950s in the former USSR, and the first working tokamak, the T-1, was developed in Russia, by physicist Natan Yavlinsky, between 1957 and 1958.

A Tokamak consists of a doughnut-shaped vacuum chamber, where the fusion happens.

The entire structure is surrounded by a series of electromagnets.

One set of magnets generates a toroidal magnetic field that forces particles to flow along the torus.

A central solenoid (a type of electromagnet), creates a second magnetic field directed along the poloidal direction, forcing the particles to follow a circular movement around the torus ‘surface.

“Toroidal direction in blue, poloidal direction in red”

To start a fusion reaction in a tokamak, first, air is expelled from the vacuum chamber.

Next, the magnets that generate the magnetic confinement field are charged up, and the fuel, in gaseous form, is introduced in the chamber.

Electric current is passed through the chamber, the gas becomes ionized, and electrons strip from nuclei.

At this point, plasma is formed, but it needs to reach temperatures between 150 and 300 million °C for the nuclei to fuse, releasing energy in the process.

Inertial Confinement: LASERS!

Inertial confinement fusion (ICF) is a bit trickier than magnetic confinement fusion.

In an ICF reactor, fusion is obtained by compressing and heating up a small fuel pellet, called “hohlraum” (which in German means “cavity”), with pulses from very powerful lasers.

“National Ignition Facility’s target chamber at Lawrence Livermore National Laboratory, Livermore, California”

Basically, an hohlraum is a hollow cylinder, usually made of gold or uranium.

Inside the cylinder there’s a fuel microcapsule, usually deuterium and tritium, arranged as a sphere to compress it uniformly from all sides.

The laser beams are not pointed towards the microcapsule itself, but at the inner surface of the cylinder.

The hohlraum’s surface absorbs radiation from the lasers and re-emits it in the form of X-rays, which causes the outermost layer of the microcapsule (made of light materials like carbon) to explode.

“Mockup of a gold-plated National Ignition Facility (NIF) hohlraum”

The explosion causes an implosion in the innermost part of the microcapsule, compressing the deuterium and tritium enough to trigger nuclear fusion.

This process is extremely complicated: any imperfection of the hohlraum’s surface will cause non-symmetrical compression of the fuel pellet and the fuel capsule must be precisely spherical for fusion ignition to start.

The most important ICF experiment is conducted at the National Ignition Facility (NIF), at Lawrence Livermore National Laboratory, in Livermore, California.

In 2022, NIF successfully ignited a fusion reaction that produced more energy than was delivered to the target, with an energy gain of about 1.5 (3.15 Megajoule produced from 2.05 Megajoule laser input).

However, while the ICF initially appeared to be a practical approach to power production, recent experiments have concluded that the efficiency of proposed ICF reactors is much lower than expected in comparison to magnetic confinement reactors.

Is fusion energy viable? Pros and cons

So, to conclude, is the fusion reactor viable?

In short, yes, specifically, it’s complicated.

Nuclear fusion doesn’t release harmful substances like carbon dioxide or other greenhouse gases. Its main by-product is helium, a non-toxic gas.

Also, nuclear reactors do not produce any type of nuclear waste and a Chernobyl-like incident is impossible for a magnetic confinement reactor.

If, by any chance, the magnet system fails, the plasma will expand and cool within seconds, stopping the reaction.

Regarding the availability of fuel, deuterium is present in abundance in every from of water, while for tritium, things are more complicated.

Tritium is radioactive, with a half-life of about 12 years, so it is not found in large quantities in nature.

A good replacement for tritium may be Helium-3, which is not present in abundance on Earth, but could be on the Moon, due to solar-wind exposure of the lunar surface.

Thus, in the near future, helium-3 could possibly be extracted from the Moon.

Tritium could also be produced by interactions of fusion neutrons with lithium, which would fulfil our needs for millions of years.

In the end, for now, the artificial ignition of a fusion reaction requires an energy input so high that the energy output is almost negligible.

We don’t really know if nuclear reactors will be commercially viable in the future, but for sure they represent a hope that it’s worth considering, since the current climatic situation.

 

Hello, world! I'm Edoardo Cignitti, a passionate enthusiast of computer science, physics, and aviation. I have an insatiable curiosity about the world and love understanding why things happen, which is why I'm particularly drawn to physics, with a keen interest in nuclear and quantum physics. I also have a soft spot for sci-fi films and enjoy playing board games. I'm excited to share my interests with you here on Let's Blog!